Respiratory protective devices have been in use in the workplace for many years, particularly at work sites where sufficient engineering or work practice controls have not been possible or feasible. Governments have established regulations and monitored conditions. Although the regulations are designed toward giving maximum protection for the worker under a variety of conditions, investigations show that respirators generally are worn by a comparatively low percentage of those in need of respiratory protection. It has been found that this is principally due to a generalized discomfort experienced by workers who wear respirators. Frequently, respirators are worn only intermittently, primarily when air contamination is severe. As a consequence, the effectiveness of respirators in reducing work-related illness is much less than it should be. To a large extent, the lack of respirator wear is due to a need for design improvement.
Among the reasons which have been found leading to respirator discomfort and nonuse are that known respirators contain excessive dead space, that known respirator devices tend to exert undue pressure on sensitive areas of the face or inside the mouth, that known respirators leak, and that known valves in respirator devices deteriorate quickly.
The dead space problem is perhaps the most difficult. Respiratory dead space is the volume of air between a face mask and the face of the individual wearing the respirator. When the wearer exhales, part of the exhaled air is trapped inside the mask. During subsequent inhalation, the trapped air is rebreathed and enters the lungs first before any fresh air. If the dead space is large enough, it is possible that only the air inside the mask will be rebreathed. Because the partial pressure of carbon dioxide exhaled is higher than the partial pressure of carbon dioxide in atmospheric air, the reinhaled air which enters the lungs first contains elevated levels of carbon dioxide. This air mixes with the residue air remaining in the lungs during exhalation, and consequently, raises the level of retained carbon dioxide in the lungs.
One basic physiological response to increased carbon dioxide retention is hyperventilation, which is an increase in the minute volume of air breathed without a corresponding increase in metabolic activity, that is, work. Minute volume is the total volume of air inhaled in a minute. Minute volume is a function of tidal volume, which is the amount of air inhaled and exhaled in one breath and the number of breaths per minute. Initially, tidal volume increases to about 70% of the vital capacity of the person. Thereafter, breathing frequency rises.
If dead space is large enough, the partial pressure of carbon dixoide in the lungs can equal the venous partial pressure of carbon dioxide. At this point, diffusion ceases, and oxygen transfer between the lungs and blood stream is prevented. Continued metabolic demands increase arterial partial pressure of carbon dioxide. This stimulates chemical receptors in the brain and triggers muscular expansion of the lungs. As lung volume increases, more fresh air is inhaled, which mixes with the dead space air and reduces the total partial pressure of carbon dioxide entering the lungs. This compensatory mechanism is effective up to a concentration of carbon dioxide of about two percent of total volume by air. Further increases in carbon dioxide concentration, dependent upon time as well, produce symptoms of fatigue, dizziness, headache, ringing in the ears, drowsiness, paralysis of the respiratory center, and finally asphyxiation and death. The United States government has set limits of carbon dioxide concentration dependent upon exposure time as follows: 2.5% for 30 minutes or less, 2% for one hour, 1.5% for two hours, and 1% for four hours.
Some stability of this mechanism may be achieved while wearing a respirator under sedentary conditions. However, as exercising increases, the increased effort of the body to reequalize adds to the body work load. If the rate of exercising changes rapidly, the problem is magnified further.
Another physiological response to increased carbon dioxide retention is cardiac stress, and in particular, hypoxic pulmonary vasoconstriction. As the partial pressure of carbon dioxide increases, the partial pressure of oxygen correspondingly decreases. When the partial pressure of oxygen in the lungs and blood stream reach 103 millimeters of mercury, hypoxic pulmonary vasoconstriction begins. At that point, oxygen transfers between the lungs and the blood stream ceases. Because the body still needs oxygen, it will start using oxygen reserves in the blood. As this happens, the vascular walls contract, causing higher blood pressure.
Still another effect of increases retained carbon dioxide is diminishing capacity to perform work. Because the partial pressure of oxygen decreases as the partial pressure of carbon dioxide increases, total oxygen intake going to the lungs is decreased. As a result, the respirator wearer has to breath more air than he would without a respirator. Because every individual has a finite, maximum capacity of air which may be inhaled, the air required to compensate for the respirator is not available to support the increased metabolic demands of work.
Thus, it is clear that if workers are going to receive the benefit of wearing a protective respiratory device when it is otherwise unsafe not to do so, that the device must be designed with an effective dead space which does not lead to an excessive carbon dioxide concentration and the resultant discomfort and other problems mentioned. Known devices have not been effectively designed with this problem in mind.
With respect to the face or mouth discomfort problem, many protective devices are designed to include a mouthpiece which is placed inside the wearer's mouth. A common result with such devices is that the strap or other device which holds the respirator to the person's head draws or pulls the mouthpiece into the person's gums and teeth. Over a period of time, the gums are irritated and the teeth move which results in discomfort, poor bite, and other mouth problems.
A further problem which leads to workers deciding not to wear respiratory protective devices is the ineffective harness design. Considering the types of protective devices having a mouthpiece received within the mouth, such leakage occurs at the interface between the lips and the mouthpiece. A common time when such leakage occurs is when the wearer moves his jaw so that the bottom lip separates slightly from the mouthpiece.
Worker discomfort is also compounded by mask design and, in particular, inhalation valve flaps whose shapes are quickly distorted with use. Even the position of the mounting affects performance. Both conditions result in increased dead space and breathing resistance.
The present invention addresses these problems with structure designed to minimize the problems and which leads to greater comfort and safety for the wearer, and consequently a greater likelihood that the wearer will wear the protective device and realize its benefits.